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Geo,paleomagnetism

1.
MAGNETICS
Introduction
Geo & Paleo
Magnetism

2.
Role of Geo & Paleo in
Geophysics & Geology
Like other geophysical methods, magnetism is
also divided into Applied & Paleo areas.
• Geomagnetism deals with the exploration of
minerals, basement under sedimentary
column (oil industry), salt domes, igneous
bodies in the subsurface, groundwater in
igneous terrain.
• Paleomagnetism deals with the history of
magnetic poles/polarity, history of rocks, and
plate tectonics.

3.
Features of a magnetic Bar
• Magnetic bar has two magnetic poles:
S
N
North pole or +ve pole, and South pole or –ve pole.
• The poles are located 1/12 of bar length inside from the end.
• Magnetic bar is surrounded by a magnetic field
produced by magnetic lines of force which flow from
north to south pole.
• If an iron needle goes in the magnetic field or touches
the pole, the needle is magnetized and starts behaving
as a tiny magnet.
• Bar magnet behaves as a Line magnet.

4.
Magnetic Parameters
• Magnetic moment(M)=2L*m,
•
•
•
•
•
where ‘L’ is half the distance between two poles, ‘m’ is the
magnetic poles.
If ‘L’ is reduced to infinitesimal ds, then bar is converted into
Magnetic Particle, its magnetic moment is M= m*ds,
If magnetic bars are placed end-on or side by side, then
M=n*m*2L
Unit magnetic pole when is placed 1 cm away from similar pole,
is acted upon by a force of 1 dyne.
Unit pole is also equivalent to 4π (12.5667) lines of force.
A magnetic pole of strength ‘m’ will generate 4π*m lines of
force.

5.
Magnetic Permeability
• Lord Kelvin defined Permeability as the ease
with which a magnetic flux can be established
in a body.
• Permeability is a ratio between the magnetic
flux through a unit cross sectional area of a
body and the flux through a like unit cross
sectional area of the air.
µ=(φ/A)/H= B/H --------- µ for space (air) is
4π*10‾7 and is indexed as 1.

6.
Magnetic Susceptibility
• This is an ability of a magnetic material
to be magnetized. The intensity of
magnetization ‘I’ depends also on
magnetizing field ‘H’.
• The intensity of magnetization, I, is
related to the strength of the inducing
magnetic field, H, through a constant
of proportionality, k, known as the
magnetic susceptibility.

7.
Magnetic Lines of Force Associated
with Magnetic Dipoles
• The force associated with this
fundamental element of magnetism, the
magnetic dipole, now looks more
complicated than the simple force
associated with gravity. Notice how the
arrows describing the magnetic force
appear to come out of the monopole
labeled N and into the monopole labeled S

9.
Earth’s magnetism: due to electric currents in liquid outer core
11.5º
magnetic dipole
[the source producing the magnetic
field is far from where were are
measuring its field; within the core]
Magnetic field is measured (units): in Teslas [T, too large]  nanoTesla (nT, 10-9 T)
9

12.
Magnetic Field Nomenclature
• At any point on the Earth's surface, the magnetic
field, F*, has some strength and points in some
direction. The following terms are used to describe
the direction of the magnetic field.
• Declination - The angle between north and the
horizontal projection of F. This value is measured
positive through east and varies from 0 to 360
degrees.
• Inclination - The angle between the surface of the
earth and F. Positive inclinations indicate F is pointed
downward, negative inclinations indicate F is pointed
upward. Inclination varies from -90 to 90 degrees

20.
lines of magnetic field intersect
the Earth’s surface at an angle
magnetic latitude (inclination) & direction to north (declination)
are easily found, but magnetic LONGITUDE can not be deduced due to the symmetry of the magnetic field about its axis
magnetic declination: [at a certain location] the difference (angle)
between geographic/true north and magnetic north [azimuth
of horizontal component of magnetic field] (degrees east or
west of true north)
magnetic inclination: [at the Earth’s surface] the angle between the magnetic field
21
and the horizontal (degrees, +90º to 0º to –90º)

21.
Fundamentals of Magnetism
magnetic dipole created by rotation of an electron around atom’s nucleus
[dipole is presented by an arrow pointing NS]
non-magnetic material: atoms tilt all different ways, so dipoles cancel each other out [zero net magnetization]
permanent magnet:
dipole lock into alignment, so they add to each other and produce a strong cumulative
magnetization
non-magnetic material
permanent magnet
22

22.
Magnetism of Rocks
remanent magnetisation: the ability to retain magnetisation in the
absence of a field or in the presence of a different magnetic field
alignment and hence magnetisation
disappears as soon as the field is removed
directions of the magnetisations of the
magnetic atoms spontaneous align
[iron materials & its compounds, e.g. magnetite]
• Most rocks contain some ferromagnetic minerals [compounds of iron]
• the atomic magnets of tiny ferromagnetic crystals or grains are aligned
along one of the crystallographic directions (called easy axes) and the
grains have strong magnetisation for their size
• if a magnetic field is applied the individual grain magnetisations will each
tend to rotate into an easy axis closer to that of the field and in this way
obtain a remanence
23

24.
 one way to magnetise a rock is by
applying a magnetic field
 another way is by heating
[usually to demagnetise a rock sample]
Curie & blocking temperatures
progressive thermal demagnetisation
Blocking temperature:
a range of temperatures characteristic
for individual minerals to be thermally
demagnetised
Curie temperature or Curie point
for a specific material: temperature
above which the material becomes
paramagnetic
[the individual atomic magnets cease to align with
one another, and the spontaneous magnetisation
25
necessary for ferromagnetism disappears]

29.
Earth’s magnetic field
• 90% is coming from internal dipolar source.
• The remaining 10% of the magnetic field cannot be
explained in terms of simple dipolar sources. It is
attributed to external solar activity,
• Complex models of the Earth's magnetic field have
been developed and are available.
• If the Earth's field were simply dipolar with the axis
of the dipole oriented along the Earth's rotational
axis, all declinations would be 0 degrees (the field
would always point toward the north). As can be
seen, the observed declinations are quite complex

37.
Secular Variation: the slow, somewhat irregular, change in the direction of the magnetic field
every few years updated maps are
produced [International Geomagnetic
Reference Field] to give both the
updated declination and its rate of change
38

38.
Magnetism of Rocks
remanent magnetisation: the ability to retain magnetisation in the
absence of a field or in the presence of a different magnetic field
alignment and hence magnetisation
disappears as soon as the field is removed
directions of the magnetisations of the
magnetic atoms spontaneous align
[iron materials & its compounds, e.g. magnetite]
• Most rocks contain some ferromagnetic minerals [compounds of iron]
• the atomic magnets of tiny ferromagnetic crystals or grains are aligned
along one of the crystallographic directions (called easy axes) and the
grains have strong magnetisation for their size
• if a magnetic field is applied the individual grain magnetisations will each
tend to rotate into an easy axis closer to that of the field and in this way
obtain a remanence
39

39.
Mineral magnetism
Magnetic susceptibility (χ): the ability of a rock to become temporarily magnetised
while a magnetic field is applied to it
paramagnetic materials  become magnetised only when the field is present
ferromagnetic materials  increase their magnetisation while a field is applied
}
this temporary magnetisation is
called induced magnetisation
strength of the magnetic field
40

40.
Different types of remanent magnetisation
Thermal Remanent Magnetisation (TRM)
as magma cools it passes through the Curie temperature as the atomic magnets of magnetic material grains align spontaneously to form one or more
magnetic domains. As the rock cools through its range of blocking temperatures, a net magnetisation is “frozen” in
 strong magnetisation
Chemical Remanent Magnetisation (CRM)
chemical alteration of a nonmagnetic iron mineral into a magnetic one, e.g. weathering, or precipitating iron oxides usually
haematite from water percolating through the rock, example: cement in sandstones forming “red bed”)
 because the process leads to haematite formation which is magnetically weak, CRM leads to weak,
though measurable, magnetisation
Detrital or Depositional Remanent Magnetisation (DRM)
as existing magnetised grains are deposited (rock erosion products, e.g. basaltic lava) together with other material to form a waterlain sediment, they tend to align their magnetisations with the field, like tiny compass needles, as they settle through the water
 weak; cases where the direction of DRM may not align closely with the inclination of the Earth’s field due to turbulence in the
depositional flow and more importantly because flattened grains tend to land flat on the floor as pieces/flakes of paper settle
through the water
Viscous Remanent Magnetisation (VRM)
if it happens that thermal fluctuations (ambient temperature) taking place over long periods of time are not too far from any rock
blocking temperatures, the rock is remagnetised in the direction of field at the time
 slow, partial magnetisation (like a compass needle in very thick oil), is very common in rock samples and is removed by
reheating to 100-220ºC
41

41.
Thermal Remanent Magnetisation (TRM)
 one way to magnetise a rock is by
applying a magnetic field
 another way is by heating
[usually to demagnetise a rock sample]
when a magma cools (solidifies including the formation of grains
of magnetic minerals) it passes through the Curie temperature as
the atomic magnets of magnetic material grains align spontaneously
to form one or more magnetic domains. As the rock cools through
its range of blocking temperatures, a net magnetisation is “frozen” in
 resulting in thermal remanent magnetisation (TRM)
Measuring reheating temperatures
• an igneous rock had originally a primary remanence
• reheated by an intrusion: if above its highest blocking temperature  all its primary remanence will be demagnetised and the rock
remagnetises in the Earth’s field at that time giving wrong remanence for the initial formation age of the rock (none awareness of
the intrusion thermal effect)
• reheated by an intrusion: not sufficient temperature to destroy all primary remanence  primary remanence is retained and a
second remanence is added as the rock cools
• Natural Remanent Magnetisation (NRM) :
progressive demagnetisation by reheating
remanence of a rock sample regardless of
how it is magnetised will be a mix
(vector  strength & direction) of the two
remanences
42

42.
Paleomagnetism: the magnetism of a rock acquired
long time ago, often when they are formed [provides inclination, declination of the location where the rock was formed]
measurements have shown that when the secular
variation is averaged over ten thousand years or
more, it coincides with the direction of the rotation
axis, and so with the true north;
this simplifies paleomagnetic interpretations
 present field: normal or N-polarity
 opposite field: reversed or R-polarity
[there have been times during the Earth history when the
magnetic poles have been interchanged]
43

43.
Paleomagnetism: measuring a paleomagnetic direction
rock-sample at a location at present magnetic equator
oriented samples (azimuth & dip recorded)
are needed [6-8 samples taken from the same
rock formation at some distance to reduce errors]
(2) does not parallel the
Earth’s present field
[exhibits a declination
angle]
laboratory measurements: (paleo-inclination &
paleo-declination)  spinner magnetometer
(1) its inclination is not 0º (as expected for its magnetic equator location)
44

44.
Magnetostratigraphy:
changes of magnetic field direction (normal/reverse) leave their records in the rocks and are used to establish
a stratigraphic order or even to date the rocks
45

50.
lines of magnetic field intersect
the Earth’s surface at an angle
magnetic latitude (inclination) & direction to north (declination)
are easily found, but magnetic LONGITUDE can not be deduced due to the symmetry of the magnetic field about its axis
magnetic declination: [at a certain location] the difference (angle)
between geographic/true north and magnetic north [azimuth
of horizontal component of magnetic field] (degrees east or
west of true north)
magnetic inclination: [at the Earth’s surface] the angle between the magnetic field
51
and the horizontal (degrees, +90º to 0º to –90º)

52.
Apparent Polar Wander (APW) paths (3)
2 alternative explanations
“true polar-wander” model:
continent is fixed, so to explain polar-wander
paths, the magnetic pole must move substantially
the magnetic pole does move a little, but it
never strays very far from the geographic pole
continental-drift model:
magnetic pole is fixed near the geographic pole,
and the continent drifts relative to the pole
53

54.
Apparent Polar Wander (APW) paths & relative continental movements
Europe & North America
280-180 Ma: APW paths coincide as Europe and North
America moved together as a unit when both were part
of Pangea. When Pangea broke up, they began to
develop separate paths
Europe & Siberia
APW paths  can show that a single mass is
formed from smaller parts: Europe & Siberia
similar APW paths back to Triassic, but differ
55
for older ages as the two collided in the Triassic

55.
Paleomagnetic Directions
cluster of directions:
replace by an average, or mean,
direction, plus an error
α95 confidence limit [statistic]:
a cone with this half-angle has a
95% probability of containing the
true direction of magnetization
α95 circle of confidence [on stereonet]
56

56.
Paleopoles: paleolatitudes & rotations
rock-sample present location: 10ºN
paleomagnetic lab-measurements: declination=20º
inclination=+49º  tan I = 2 tan λ  paleolatitude=30ºN 
apparent-North pole was at 60º (90º-λ) away from rock location
apparent North pole:
relative to our rock
sample
Paleopole is found: by traveling 60º
around the Earth along a great circle,
starting from the present rock location,
in the direction of declination, 20º
different from present pole  rock has moved
different declination  rock has rotated about
a vertical axis
different inclination  moved N-S or tilted
but cannot say is the rock has
changed its paleo-longitude
[due to axial symmetry of the dipole field]
57

57.
Apparent Polar Wander (APW) paths & relative continental movements
APW paths can show if there has been relative movement between land masses
[provided the APW paths cover the same time span]
58